Acinic cell carcinoma (ACC) is an uncommon neoplasm of the salivary glands with low malignant potential. Even rarer is the occurrence of primary ACC of the breast, there is only one previously documented case. We now describe the second case of a primary carcinoma of the breast with morphological, ultrastructural, and immunohistochemical features consistent with an ACC. In contrast to the previous case report we report the lack of encapsulation of the tumor and a spectrum of cell types within the tumor including cells resembling well-differentiated infiltrating ductal carcinoma. Additionally we did not observe the abundant nuclear abnormalities noted in the previously published case. We report for the first time the mucicarmine-staining pattern of this tumor as well as immunohistochemistry directed against cytokeratin 7, the estrogen receptor, and the progesterone receptor.